Sparse-code multiple access (SCMA) is a non-orthogonal multiple access scheme that allows multiple devices, users, or user equipments (UEs) to share channel resources in the code and power domain. Potential transmit devices are allocated time and frequency resources, also referred to as resource units.
SCMA is a codebook-based non-orthogonal multiplexing technique realized by the super-imposing of multi-dimensional codewords selected form SCMA codebooks. Instead of spreading of QAM symbols as in code division multiple access (CDMA) and low density signature (LDS), coded bits are directly mapped to multi-dimensional sparse complex codewords. One major benefit of SCMA codebooks is the shaping gain of multi-dimensional constellations over repetition coding of LDS spreading. SCMA is classified as a waveform/modulation and multiple access scheme. SCMA codewords are laid over multi-carrier tones such as OFDMA tones. In SCMA overloading is achievable with moderate complexity of detection thanks to the sparseness of SCMA codewords. SCMA can show noticeable gain over LDS especially for larger constellation sizes where the gain of constellation shaping is potentially larger. SCMA also provides system advantages due to its spreading and overloading capabilities. Interference whitening, open-loop user multiplexing and massive connectivity are some examples showing the benefit of SCMA from system point of view.